1. Field of the Invention
The present invention relates to a backlight unit of a liquid-crystal display module that is built into a portable personal computer such as a notebook-size personal computer (PC).
2. Description of the Related Art
In recent years, there is an increasing requirement for displaying videos and images captured with a digital camera, with high brightness, on a liquid-crystal display (LCD) module that is built in a notebook-size personal computer, or a laptop personal computer (hereinafter referred to as a laptop computer).
FIG. 1 is a perspective diagram of a simple optical waveguide backlight unit according to a conventional technique. A backlight unit 100 shown in FIG. 1 includes: an optical waveguide 101 having no prism; a fluorescent (FL) tube 102 as a light source; a lamp reflector 103 that accommodates the FL tube 102, reflects light emitted from the FL tube 102, and directs the reflected light toward the optical waveguide 101; a lamp holder 104 that accommodates the lamp reflector 103; a mold frame 105 that accommodates the optical waveguide 101 and the lamp holder 104; and an optical sheet 120 consisting of a lower diffusion sheet 106, a first prism sheet 107 of a refractive deflection type that deflects light to a normal direction of the surface of the optical waveguide 101, a second prism sheet 108, and an upper diffusion sheet 109. In the simple optical waveguide backlight unit 100 according to the conventional technique, a bulge like a plate screw is formed around a lower side of the optical waveguide 101 such that the surface area of the lower side of the optical waveguide 101 that faces the mold frame 105 via the lamp holder 104 is larger than the surface area of an upper side of the optical waveguide 101 that faces the optical sheet 120 and a liquid-crystal display (LCD) 130, in order to accommodate and fix the optical waveguide 101 within the mold frame 105 by engagement.
Further, there is an increasing requirement for a thin and light-weight mobile laptop computer. In order to meet these requirements, a prism optical waveguide backlight unit has come to be used for a light source of the LCD module.
FIG. 2 is a perspective diagram of the prism optical waveguide backlight unit according to a conventional technique. A backlight unit 200 shown in FIG. 2 includes: an optical waveguide 201 having a prism function; an FL tube 202; a lamp reflector 203; a lamp holder 204; a mold frame 205; and an optical sheet 220 consisting of a prism sheet 206, and a diffusion sheet 207. An LCD 230 is disposed at the opposite side of the optical waveguide 201 of the optical sheet 220. The prism optical waveguide backlight unit has brightness that is improved by 1.2 times of that of the simple optical waveguide backlight unit, and is made with two fewer optical sheets than the simple optical waveguide backlight unit. Therefore, the prism optical waveguide backlight unit can also contribute to a reduction in weight and size. However, because of the presence of a prism function in the optical waveguide, there is a limit to a method of fixing the optical waveguide in the prism optical waveguide backlight unit, as compared with the simple optical waveguide backlight unit.
When the prism optical waveguide backlight unit 200 employs the configuration of the simple optical waveguide backlight unit, that is, when a bulge is formed on a lower side of the optical waveguide 201, the following problem occurs. When the light emitted from the FL tube 202 accommodated in the mold frame 205 via the lamp holder 204 is sent directly to the optical waveguide 201 or is sent indirectly to the optical waveguide 201 after being reflected from the lamp reflector 203, the light emitted from the FL tube 202 is reflected diffusely from the bulge formed on the optical waveguide 201. Therefore, irregular brightness occurs in the display of the LCD module 230.
Consequently, at present, there is no other means to avoid the bulge from the optical waveguide 201 and connect the lamp holder 204 to the optical waveguide 201 using a double-faced adhesive tape, in order to fix the optical waveguide 201. According to the prism optical waveguide backlight unit 200, when the optical waveguide 201 is fixed insufficiently due to variations in manufacturing conditions of the optical waveguide 201, parts that are insufficiently fixed by the double-faced adhesive tape are easily peeled off by shock or the like, and the optical waveguide 201 can drop. There is a risk that the FL tube 202 can be broken by the dropped optical waveguide 201.
A liquid-crystal display device having a backlight described in a patent literature 1 has an object of providing a low-price liquid-crystal display device (LCD) having an optical waveguide of a uniform in-plane brightness distribution. The liquid-crystal display device has the backlight that accommodates the optical waveguide disposed on a lower surface of an LCD module and an FL tube disposed near one side surface of the optical waveguide along this side surface, within a case. The surface of the optical waveguide has an approximately square shape. At least one stopper provided in contact with one side of the optical waveguide at the FL tube side or provided near this one side prevents the optical waveguide from moving to the FL tube side. The other three sides of the optical waveguide are held by an inner wall of the case formed along the shape of the optical waveguide.
[Patent document 1] Japanese Patent Application Unexamined Publication No. 7-261173 (paragraphs [0009] to [0017] of the specification, [FIG. 1] and [FIG. 2], and the abstract)